The drive train serves two functions - it transmits power from the engine to the drive wheels, and it varies the amount of torque. "Power" is the rate or speed at which work is performed. "Torque" is turning or twisting force. Multiple ratio gearboxes are necessary because the engine delivers its maximum power at certain speeds, or RPM (Rotations Per Minute). In order to use the same engine RPM's at different road speeds, it is necessary to change the "Gear Ratio" between the engine and the drive wheels. Just like a bicycle, the car has to switch gears in order to move at a wide range of speeds. Unlike your bicycle, the car's drivetrain also has to allow you to back up.
There are actually two sets of gears in the drive train; the transmission and the differential. The transmission allows the gear ratio to be adjusted, and the differential lets the drive wheels turn at different speeds.
Manual transmissions usually have four or five speeds, and often have "overdrive", which means that the output shaft can turn faster than the input shaft for fuel economy on the highway. Some use an electric clutch and a switch that controls whether the overdrive is engaged or not. An interesting development on a few cars is the "clutchless" manual transmission, which uses a stick shift and an automatic electric clutch. Speed and position sensors, mini computers, and throttle controls keep the engine from over-revving when the driver shifts gears. As with many automotive "inventions", this is an old idea which may now reach feasibility due to the computer revolution.
Automatic transmissions commonly use three or four forward gears to blend speed and torque. In the case of a three-speed transmission, first gear delivers maximum torque and minimum speed for starting. Second gear offers medium torque and speed for acceleration and hill climbing. Third gear allows maximum speed with minimum torque for highway travel. A reverse gear permits backward movement.
A transmission is a speed and power changing device installed at some point between the engine and driving wheels of a vehicle. It provides a means for changing the ratio between engine RPM (Revolutions Per Minute) and driving wheel RPM to best meet each particular driving situation.
There are two types of transmissions; manual and automatic. If you have a manual transmission, you have to shift the gears yourself, usually with a stick located on your console and the clutch pedal. If you have an automatic transmission, the mechanism changes without any help from you. This is accomplished through a system that works by oil pressure. Each shift of the gears is controlled by a shift valve; the gears shift change depending on speed, the road, and load conditions.
Another basic component of all drive trains is some form of a clutch. it allows the engine to continue rotating while the gears and wheels are stationary. Automatic transmission cars use a "torque converter" in lieu of a clutch.
From the back of the engine to where the rubber meets the road, the drivetrain encompasses one of the most complicated systems of your car. Some people say looking at a transmission "makes their brain hurt".
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Automatic Transmissions
An automatic transmission is much easier to drive than a manual transmission, because you don't have to use a clutch pedal or gearshift lever. An automatic transmission does the work all by itself. The first automatic transmission appeared in 1939.
Automatic transmissions automatically change to higher and lower gears with changes in the car's speed and the load on the engine. These transmissions are also aware of how far down you have pushed the gas pedal, and shift accordingly.
The system is operated by transmission fluid pressure; shift valves control the gear changes. A "governor" controls the shifting of the gears. It's linked to the output shaft and throttle valve and controls the transmission fluid supply, at different pressures, to the shift valve. Here's how it works: the output shaft turns the governor. The faster the car goes, the faster the governor turns. Oil is sent from the pump to the shift valves by centrifugal force from the governor. The shift valves move out, and send the transmission fluid to the gear shifting mechanisms in the transmission. When you slow down, the valves move in, and send the transmission fluid in the opposite direction. This action changes the gears. Therefore, by routing the pressure to the clutches and brake bands, the different gears are selected.
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Brake Bands
A brake band is made of steel, and has a friction lining. One end of the band is attached a servo actuating rod. A servo actuating rod is a hydraulic piston (a cylinder with a piston inside it) that is open at one end to allow brake fluid to flow in. The piston is normally in the released position because it's kept that way by a spring. However, when pressurised fluid is sent to the cylinder, the fluid forces the piston forward. This causes the brake band to tighten, and this locks the brake.
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The Clutch
The clutch allows you to connect and disconnect the engine and the transmission, both starting up and during shifts. Friction plates route the rotation of the engine crankshaft to the gears, and then to the wheels. It takes the rotation up slowly, so that you aren't off to a screeching start. In a manual transmission, you disengage the clutch when you press the pedal down. The pedal works the thrust pad, and it presses levers in the middle of the clutch cover. Doing all this lifts the pressure plate away from the clutch plate. The flywheel (which is turned by the crankshaft from the transmission shaft) gets disconnected.
When you lift the clutch pedal, springs force the pressure plate and clutch plate against the flywheel. The clutch plate friction linings allow it to slide before becoming engaged. The sliding causes a smooth start instead of a jolt.
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The Clutch Pedal, Cables and Levers
One way to activate the throw-out fork of the clutch is by using a system of levers and cables. These levers and cables are connected between the clutch pedal and the throw-out fork. When you press the clutch pedal with your foot, the pressure is transmitted to the fork through the cable and lever arrangement.
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The Clutch Plate
The clutch plate is a thin, steel, disc. Its center is connected to the transmission input shaft by a grooved piece of metal, or hub. The disc is covered with material that is similar to the break linings. This material allows the clutch to slip smoothly and quietly.
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Constant Velocity Joints (CV Joints)
Front wheel drive cars need u-joints which not only allow up and down motion, but steering motion as well. the angle at which they turn requires a different design than the standard U-joint.
Constant velocity, or CV joints are universal joints that are able to transfer torque at large angles efficiently. These joints transfer power very smoothly. They are comprised of four basic parts: 1. The outer section, which has grooves machined on its inner surface, 2. the bearings, which are usually in a "cage", 3. the inner ball, which has grooves on its outer surface for the bearings to ride in, and 4. a rubber boot to protect the unit from dirt and moisture.
A common cause of CV joint failure is cracks in the CV boot. As dirt enters the CV joint, its parts grind themselves until a clicking noise is heard when turning, or until they fail completely. The boots should be replaced as soon as cracking is visible in their rubber folds.
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The Differential
The differential is the thing that works both drive axles at the same time, but lets them rotate at different speeds so that the car can make turns. When a car makes a turn, the outer wheel has to turn faster than the inner wheel, due to the difference in the length of the paths they take. The differential is located between the two wheels, and is attached to each wheel by a half-shaft rotated through a bevel gear. Four-wheel drive cars have a separate differential for each pair of wheels.
A grooved, or splined, axle side gear is positioned on the splined end of each axle. The side gears are driven by "spider" gears, which are little gears mounted on a shaft attached to the differential case. As it is supported by the differential case, the side gear can turn inside the case.
The differential case can be turned, revolving around the axle gears. The differential pinion (a pinion is a small gear that either drives a larger gear or is driven by one) shaft turns the ring gear, which is fastened to the differential case. The propeller shaft (drive shaft) connects the transmission output shaft to the differential pinion shaft. The turning differential case is mounted on two large bearing holders. These bearings are called carrier bearings.
The propeller shaft rotates the ring gear pinion, and the pinion turns the ring gear. The ring gear then turns the differential case and pinion shaft, but the axle side gears will not turn. By passing the differential pinion shaft through two differential pinion gears that mesh with the side gears, the case will turn and the axle side gears will turn with it. During turns, the side gears turn at rates dictated by the radius of the turns, and the spider gears then turn to allow the outer wheel to turn faster than the inner one.
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Differential Fluids
For lubrication fluid, a very heavy oil, must be used in rear axle housings. Special hypoid oils are used in the differential case. Even another type of fluid, or oil must be used in a positraction type differential.
The oil is circulated by the ring gear, and flung all over all the parts. Special troughs, or gullies are used to bring the oil back to certain spots, like the ring and pinion area and the piston bearings. The fluid is kept in with gaskets and oil seals. The bottom of the housing has a drain plug, and another filler plug is located part way up the housing. The housing must never be filled above this plug.
The housing fluid lubricates some of the outer bearings, but others have lubrication fittings for the injection of wheel bearing grease. A hand gun, not a pressure grease gun must be used to grease these bearings (sparingly). A pressure grease gun could inject grease into the brakes-- greasy brakes are inefficient at best!
Finally, some bearings are filled with grease at the factory and are sealed. These never require attention unless they are defective.
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The Drive Shaft
The drive shaft, or propeller shaft, connects the transmission output shaft to the differential pinion shaft. Since all roads are not perfectly smooth, and the transmission is fixed, the drive shaft has to be flexible to absorb the shock of bumps in the road. Universal, or "U-joints" allow the drive shaft to flex (and stop it from breaking) when the drive angle changes.
Drive shafts are usually hollow in order to weigh less, but of a large diameter so that they are strong. High quality steel, and sometimes aluminum are used in the manufacture of the drive shaft. The shaft must be quite straight and balanced to avoid vibrating. Since it usually turns at engine speeds, a lot of damage can be caused if the shaft is unbalanced, or bent. Damage can also be caused if the U-joints are worn out.
There are two types of drive shafts, the Hotchkiss drive and the Torque Tube Drive. The Hotchkiss drive is made up of a drive shaft connected to the transmission output shaft and the differential pinion gear shaft. U-joints are used in the front and rear. The Hotchkiss drive transfers the torque of the output shaft to the differential. No wheel drive thrust is sent to the drive shaft. Sometimes this drive comes in two pieces to reduce vibration and make it easier to install (in this case, three U-joints are needed).The two-piece types need ball bearings in a dustproof housing as center support for the shafts. Rubber is added into this arrangement for noise and vibration reduction.
The torque tube drive shaft is used if the drive shaft has to carry the wheel drive thrust. It is a hollow steel tube that extends from the transmission to the rear axle housing. One end is fastened to the axle housing by bolts. The transmission end is fastened with a torque ball. The drive shaft fits into the torque tube. A U-joint is located in the torque ball, and the axle housing end is splined to the pinion gear shaft. Drive thrust is sent through the torque tube to the torque ball, to transmission, to engine and finally, to the frame through the engine mounts. That is, the car is pushed forward by the torque tube pressing on the engine.
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The Flywheel
The flywheel is a fairly large wheel that is connected to the crankshaft. It provides the momentum to keep the crankshaft turning between piston firings.
The flywheel is the base for the entire clutch attachment. The side of the flywheel that the clutch is attached to is smooth, so that it provides a surface for friction. The clutch assembly is mounted to the flywheel, sandwiching the clutch plate in between. A bearing, called the "pilot bearing" is installed in a hole in the center of the flywheel. This lubricated bearing, either a ball bearing or a bronze bushing, is used to support one end of the clutch shaft, which is also the transmission input shaft. Around the flywheel is the ring gear, which the starter motor turns when the key is turned.
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Front Wheel Drive
Many cars use a front drive axle. Most front-wheel drive axles are constructed the same way as rear-wheel drive axles, with one exception. A front-wheel axle assembly must provide a way to turn the wheels as well as drive them.
The clutch or torque converter sends the power on to the transmission input shaft. Next, the power is sent on to the differential by gears or chains (belts). It goes through the differential gears through the axle and CV Joints and finally to the front wheels.
Front wheel drive was not new in the eighties when it became popular. Front wheel drive was introduced by the Pennington Car Company in 1900. Before that, steamers and electric cars had used it for years.
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Hydraulic Clutch
Another method used to activate the clutch throw-out fork is the hydraulic clutch. This method is often used when the mechanical design of the car makes it difficult to use levers and cables. It is also used to multiply force, reducing driver fatigue.
With a hydraulic clutch, when you press the clutch pedal, it moves a small cylinder called the "master" cylinder. Pressure is created in the master cylinder which is, in turn, transmitted to the "slave" cylinder. The slave cylinder is attached to the throw-out fork by a small adjustable rod, so when pressure is exerted on the slave cylinder, it operates the fork. Both master and slave cylinders are designed in such an uncomplicated way that they are easy to attach with hydraulic tubing.
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Manual Transmission
The manual transmission provides a means of varying the relationship between the speed of the engine and the speed of the wheels. Varying these gear ratios allows the right amount of engine power at many different speeds.
Manual transmissions require use of a clutch to apply and remove engine torque to the transmission input shaft. The clutch allows this to happen gradually that so that the car can be started from a complete stop.
Modern manual transmissions do not disengage any of the forward drive gears, they are simply connected to their shafts through the use of "synchronizers". Reverse is achieved through reverse idler gears, which are engaged to move the car backwards.
Some manual transmissions have an "overdrive." An overdrive is a mechanical unit bolted to the rear of the transmission. It is usually known as fifth gear. When you use it, it will reduce the engine speed by about one-third, while maintaining the same road speed.
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Positraction Differentials (LSD)
A positraction differential or limited slip differential (LSD) is a special traction differential. Its purpose is to improve the way your differential performs under adverse conditions. When one wheel starts to slip, these differentials transfer the torque to the wheel that is not slipping. The car can then continue to go forward. There are several different kinds of positraction differentials, but all of them are based on a friction device to provide resistance to normal differential operation.
A positraction differential provides better traction, which is handy when roads are slippery. It also lends itself to fast acceleration.
One type uses four differential pinions instead of two, with two pinion shafts. It also uses a series of four clutch discs. The differential pinions run into resistance when they try to turn the axle side gears. The resistance gets transferred to the pinion shafts driving the pinions. The shafts are forced to slide up little ramps. This action moves both shafts outward. The pinions cause the clutches to lock.
Other types use cone clutches, or disc clutches under pressure from coil springs. By restricting the differential action, torque is delivered to the slipping wheel.
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Torque Converter
The torque converter is a type of fluid coupling between the engine and the gearbox to even out speed changes. The torque converter also multiplies engine torque.
The torque converter is used as a clutch to send the power (torque) from the engine to the transmission input shaft. It has three parts; an impeller connected to the engine's crankshaft, a turbine to turn the turbine shaft which is connected to the gears, and a stator between the two. The torque converter is filled with transmission fluid that is moved by the impeller blades. The stator's vanes catch the oil thrown off from the impeller, and use it to move the turbine's blades. When the impeller spins above a certain speed, the turbine spins, driven by the impeller.
In some designs, the torque converter locks the impeller and the turbine together when at highway speeds, which increases efficiency.
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Transmission Gears
Most cars have from three to five forward gears, and one reverse gear. The transmission changes the ratio of the engine speed and the wheels by connecting gears in various combinations. If a gear with 10 teeth is driving a gear with 20 teeth, the drive would be said to have a 2:1 ratio.
First gear connects the engine power to the drive wheels via a pair of reduction gear sets, which gives increased power and reduced wheelspeed when the car is beginning to move. This means the engine is turning much faster than the output shaft, typically around a 4:1 ratio. Intermediate speeds are delivered by changing the gear ratio closer to 1:1. Final drive is usually accomplished by directly linking the input and output shafts, giving a 1:1 gear ratio. Using a moveable set of different sized gears, it's possible to get several degrees of torque output. The differential pinion, driven by the drive shaft, turns the ring gear, which acts like a single speed transmission. This further reduces RPM's and increases torque by a set ratio.
Gears work exactly like levers. A small gear driving a larger one gives an increase in torque, and a decrease in speed, and vise-versa.
Transmission gears are heat-treated, high quality steel. They have smooth, hard teeth, cut on precision machinery while red hot. There are many types of gear teeth, but most transmissions use spur and helical gears. Most of the gears are the helical type, because they last longer and are more quiet than spur gears. There has to be enough room (a few thousandths of an inch) between the gear teeth for lubrication, expansion, and any irregularities in size.
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Transmission Oil
The transmission needs lubrication to keep all of the gears and shafts running smoothly. This is accomplished by partially filling the transmission housing with thick transmission gear oil. When the gear gears spin, they fling the fluid around and lubricate all of the parts. Oil seals at the front and rear stop the fluid from leaking out of the housing.
Fluid levels should be checked when you change your oil, or if you notice difficulties or differences in shifting. This can indicate that the level of fluid might be low.
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Transmission Fluid
Transmission fluid is a special kind of oil used only for transmissions. It circulates through and lubricates the gears. Check your car's owner's manual for the type to use. No other type of oil should ever be used in your transmission.
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2WD, 4WD and AWD
2-Wheel Drive
The engine, clutch and gearbox are usually mounted on the frame at the front of the vehicle. The rotating motion produced by the crankshaft at the front of the vehicle is transmitted either to the two wheels at the rear (rear wheel drive), or the two wheels at the front (front wheel drive). Some cars are manufactured with rear mounted engines that drive the rear wheels, and front mounted engines that drive the front wheels.
4 Wheel Drive
4-wheel drive vehicles use live front and rear drive axles. When the front drive axle receives power from the transfer case, along with the rear drive axle, the vehicle can function well on off-road terrain (sand, rocks, mud, snow, etc.). A 4-wheel drive vehicle has one drive axle that is automatically in use. The operator of the vehicle has to activate and deactivate the second live drive axle.
All Wheel Drive (AWD)
All-wheel drive vehicles use live front and rear drive axles. When the front drive axle receives power from the transfer case, along with the rear drive axle, the vehicle can function well on off-road terrain (sand, rocks, mud, snow, etc.). A 4-wheel drive vehicle has one drive axle that is automatically in use. The operator of the vehicle has to activate and deactivate the second live drive axle. An all-wheel drive vehicle has both axles live at all times without manually activating or deactivating axles.
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The Universal Joint (U-joint)
The Universal joint (U-joint) is used to connect the drive shaft to the transmission output shaft and the differential pinion gear shaft. This joint must be flexible enough to allow changes in the driving angle (road incline) and the drive shaft. This way, the torque is constantly transmitted when the rear axle is moving up and down. Smaller U-joints are used to route the turning motion of the steering wheel through the steering column to the steering box.
There are two types of U-joints, the cross and roller type and the ball and trunnion type. The cross and roller type is used the most; it allows the drive shaft to bend. The ball and trunnion type less frequently used; it allows the drive shaft to bend and also permits backward and forward motion of the drive shaft.
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